Let$i$ be the current through the arm $ADC$. Then current through the arm $ABC=\left(2.1-i\right)$. As there is no deflection in the galvanometer, the two arms are parallel to each other, therefore their potential drop across the arm will be same. Hence,
$\left(20+5\right)i=\left(8+2\right)(2.1-i)$

$$\begin{gathered} \Rightarrow 25i=21-10i \\ \Rightarrow 35i=21 \end{gathered}$$
$\Rightarrow i=\frac{21}{35}=\frac{3}{5}=0.6\text{A}$